Various one-trip, window-milling systems have been used in the past. In U.S. Pat. No. 5,109,924, issued to Jurgens, a pilot mill is combined with a series of watermelon mills for use in conjunction with a whipstock to mill a window. This system is not made for thru-tubing applications. In U.S. Pat. No. 5,445,222, a whipstock is separately installed on tubing and the tubing is removed. An assembly is then installed on tubing, which comprises a pilot mill with a tapered cutting surface, followed by subsequent stages of spiral-type mills which are progressively larger as they get more remote from the pilot mill. Wear pads are randomly distributed above and below each of the spiral mills. The pilot mill is formed into a cone-shaped head. The assembly is rotated from the surface while drilling fluid is circulated through the drill-string which rotates the mills. Both these techniques require the additional time to pull the production tubing. A thru-tubing application, particularly that runs on coiled tubing which cannot itself be rotated, requires the use of a downhole motor to rotate the milling assembly.
In the past, cement plugs have been used in conjunction with bent subs installed through tubing to try to mill a window. Thus, for example, in a 7" cased hole with a cement plug, and having 41/2" production tubing, a window has been attempted using a bent sub. The system was not one-trip as the initial mill had to be replaced before it made sufficient penetration. The problem in the past has been that the torque output of the downhole motor presents a limiting factor on the milling operation. The result in past attempts to conduct milling with a downhole motor, in conjunction with a whipstock, have led to unacceptable penetration times coupled with the need to change pilot mills which wore prior to making a full penetration.
Thus, use of mills with sharp angles, which are suitable for systems operated with tubing rotated from the surface, become impractical when the driver is a downhole motor.
Accordingly, what is needed is a truly one-trip system that can operate through tubing in conjunction with a whipstock that can be inserted through tubing where the window can be milled in a single trip using a downhole motor as the driver. The apparatus and method that has been developed allows for smooth operation which is an aid in improving the penetration rate. The configuration of the starter mill, and how it is disposed with respect to the whipstock, are also factors in reducing the milling time required for the penetration of the initial mill into the casing. Apart from the configuration of the starter mill and proper facilities to orient it at the casing, another factor that contributes to the objective of the invention is the sharing of the milling job to make the complete window between the starter mill and subsequent mills, which help stabilize the assembly. Additionally, the specific design of the orientation device for the starter mill also significantly adds to the smoothness of its operation, minimizes cutter breakage, and facilitates the return of circulating fluid and cuttings to the surface. These features, singly and together, contribute to the objective of the present invention which is to provide a one-trip, thru-tubing system that can operate in conjunction with a whipstock to make a window in the casing in a single trip in a short amount of time, using a downhole motor as the driver.
In the past, when attempts have been made to mill an exit through a casing through tubing, using a downhole motor, a one-trip system was not possible. The reason was that the initial mill could not complete the milling operation sufficiently to allow the second mill to finish the hole. In the past, the initial mill had to be pulled out of the hole and replaced before the window was milled sufficiently large to allow the secondary mill to enter and finish the window. One of the reasons this occurred was that the initial mill was unstable and tended to initially jam. Additionally, when running on coiled tubing and pressurizing the coiled tubing, the result which occurred was an elongation of the tubing. To compensate for such an elongation, operators would lift up on the assembly so that the downhole motor would not start the initial mill when the initial mill is jammed between the face of the whipstock and the casing. To compensate for the elongation of the coiled tubing, the initial mill would be started when it was in a position well above the whipstock. Prior attempts to mill through tubing have used aggressive designs for the initial mill, such as shown in FIGS. 1 and 2. Those designs could strike the top of the whipstock, which could break cutters off. The mill would stick and stall the downhole motor, which could not put out sufficient torque to maintain rotation of such aggressively designed mills. The stalling of the mill, combined with applying a pickup force at the surface which would get the mill rotating, followed by once again lowering the mill into cutting position, put severe loads on the cutting structure of such mills and, as a result, the tungsten-carbide cutters would be broken off, greatly contributing to the early failure of the starter mill. Thus, one of the objectives of the present invention is to combine a more gradual angle in the starter mill, in combination with a stabilizer or guide which allows the mill to start when it is already in the vicinity of the whipstock after pressurization of the coiled tubing and the resultant expansion. Jamming and stalling are minimized, which promotes the useful life of the mill. Another objective is to provide, in a small confined space designed for thru-tubing, an initial mill that has a multiplicity of rows of cutters so that even if some break off, the rate of milling is not dramatically adversely affected. Thus, for example, in a mill having an outside diameter as small as about 3", as many as 12 rows of cutters can be used alongside of the lead mill.